Since residual fuel consumption is continuously decreasing, there is a pressing need to find a way to convert high-residual gas oil feedstock into more-marketable products, such as diesel fuel and gasoline. Proposed alternatives include several fixed-bed catalytic treatments, such as the following:
U.S. Pat. No. 4,102,779 relates to a process for hydrotreating hydrocarbons containing metals and asphaltenes. It provides a first stage of demetallization of feedstock in the presence of a macroporous catalyst on a silica-gel matrix, and a second stage of desulfurization of feedstock in the presence of another suitable catalyst.
U.S. Pat. No. 4,048,060 also presents a two-stage hydrotreatment process which initially uses a microporous catalyst for desulfurization, and thereafter employs a larger pore diameter catalyst for subsequent demetallization.
U.S. Pat. Nos. 4,166,026 and 4,191,636 refer to one- or two-stage hydrotreatment processes (for heavy hydrocarbons with high asphaltene and metal contents) consisting of hydrometallization and selective catalytic cracking of asphaltenes in a first reaction zone, and then hydrodesulfurization of products in a second reaction zone.
On the other hand, catalysts have been developed to attempt to improve demetallization and hydrodesulfurization processes, as evidenced by U.S. Pat. No. 4,328,127 (which relates to a combination of Co and Mo supported on alumina with a specific pore volume and distribution), and by U.S. Pat. 3,630,888 (which refers to a microporous structure having access channels interstitially distributed therethrough).
In the foregoing processes fairly severe operation conditions (pressures above 1500 psi and temperatures above 380.C) are required to attain acceptable demetallization (HDM), desulfurization (HDS), and hydroconversion (HC) levels. Even use of hydrogen partial pressures below 1800 psi causes a higher deposition of coke on an active surface and thus reduces life cycles of previously-developed catalysts to less than one year.